Flexography is a method of printing that is commonly used for the industrial production of thousands to millions of duplicate printed images. Flexography is versatile and can be employed for printing on a variety of substrates such as paper, paperboard stock, corrugated board, films, foils and laminates. Newspapers, food packaging, and grocery bags are prominent examples. Coarse surfaces and stretch films can be economically printed only by means of flexography. Flexographic printing plates are relief plates with image elements raised above open areas. Generally, the plate is somewhat soft, and flexible enough to wrap around a printing cylinder, and durable enough to print over a million copies. Such plates offer a number of advantages to the printer, based chiefly on their durability and the ease with which they can be made.
A typical photosensitive printing blank used to manufacture a flexographic printing plate is a multilayered article made of, in order, a backing, or support layer; one or more unexposed photocurable layers; optionally, a protective layer or slip film; and often a protective cover sheet.
The support sheet or backing layer lends support to the plate. The support sheet, or backing layer, can be formed from a transparent or opaque material such as paper, cellulose film, plastic, or metal. Preferred materials include sheets made from synthetic polymeric materials such as polyesters, polystyrene, polyolefins, polyamides, and the like. Generally, the most widely used support layer is a flexible film of polyethylene teraphthalate. The support sheet may also include an adhesive layer for more secure attachment to the photocurable layer(s). Optionally, an antihalation layer may be provided between the support layer and the one or more photocurable layers to minimize halation caused by the scattering of UV light within the non-image areas of the photocurable resin layer.
The photocurable layer(s) can include any of the known photopolymers, monomers, initiators, reactive or non-reactive diluents, fillers, and dyes. The term “photocurable” refers to a composition which undergoes polymerization, cross-linking, or any other curing or hardening reaction in response to actinic radiation with the result that the unexposed portions of the material can be selectively separated and removed from the exposed (cured) portions to form a three-dimensional or relief pattern of cured material.
Preferred photocurable materials include an elastomeric compound, an ethylenically unsaturated compound having at least one terminal ethylene group, and a photoinitiator. Exemplary photocurable materials are disclosed in European Patent Application Nos. 0 456 336 A2 and 0 640 878 A1 to Goss, et al., British Patent No. 1,366,769, U.S. Pat. No. 5,223,375 to Berrier, et al., U.S. Pat. No. 3,867,153 to, U.S. Pat. No. 4,264,705 to Allen, U.S. Pat. Nos. 4,323,636, 4,323,637, 4,369,246, and 4,423,135 all to Chen, et al., U.S. Pat. No. 3,265,765 to Holden, et al., U.S. Pat. No. 4,320,188 to Heinz, et al., U.S. Pat. No. 4,427,759 to Gruetzmacher, et al, U.S. Pat. No. 4,622,088 to Min, and U.S. Pat. No. 5,135,827 to Bohm, et al., the subject matter of each of which is herein incorporated by reference in its entirety. More than one photocurable layer may be used.
The photocurable materials generally cross-link (cure) and harden through radical polymerization initiated by exposure to some actinic wavelength region. As used herein, actinic radiation is radiation capable of effecting a chemical change in one or more compounds in the materials of the photocurable layer. Actinic radiation includes, for example, amplified (e.g., laser) and non-amplified light, particularly in the UV and violet wavelength regions. One commonly used source of actinic radiation is a mercury arc lamp, although other sources are generally known to those skilled in the art.
The protective layer or slip film is a thin layer that protects the photosensitive printing blank from dust and increases its ease of handling.
In a conventional (“analog”) plate making process, the slip film is transparent to UV light. In this process, the printer peels the cover sheet off the printing plate blank and places a negative on top of the slip film layer. The plate and negative are then subjected to flood-exposure by UV light through the negative. The areas exposed to the light cure, or harden, and the unexposed areas are removed (developed) to create the relief image on the printing plate. Instead of a slip film, a matte layer may also be used to improve the ease of plate handling. The matte layer typically comprises fine particles (silica or similar) suspended in an aqueous binder solution. The matte layer is coated onto the photopolymer layer and then allowed to air dry. A negative is then placed on the matte layer for subsequent UV-flood exposure of the photocurable layer.
In a “digital” or “direct to plate” plate making process, a laser is guided by an image stored in an electronic data file, and is used to create an in situ negative in a digital (i.e., laser ablatable) masking layer, which is typically a slip film which has been modified to include a radiation opaque material. Portions of the laser ablatable layer are ablated by exposing the masking layer to laser radiation at a selected wavelength and power of the laser. Examples of laser ablatable layers are described, for example, in U.S. Pat. No. 5,925,500 to Yang, et al., and U.S. Pat. Nos. 5,262,275 and 6,238,837 to Fan, the subject matter of each of which is herein incorporated by reference in its entirety. The plate and the in situ negative are then subjected to flood exposure by actinic radiation (e.g., UV light) through the in situ negative.
After imaging, the photosensitive printing element is developed to remove the unpolymerized portions of the layer of photocurable material and reveal the crosslinked relief image in the cured photosensitive printing element. Typical methods of development include washing with various solvents or water, often with a brush. Other possibilities for development include the use of an air knife or heat plus a blotter. The resulting surface has a relief pattern that reproduces the image to be printed and which typically includes both solid areas and patterned areas comprising a plurality of relief printing dots. After the relief image has been developed, the relief image printing element may be mounted on a press and printing commenced.
The relief image comprises a plurality of printing dots and the shape of the printing dots and the depth of the relief, among other factors, have an effect on the quality of the printed image. It can be very difficult to print small graphic elements such as fine dots, lines and even text using flexographic printing plates while maintaining open reverse text and shadows. In the lightest areas of the image (commonly referred to as highlights) the density of the image is represented by the total area of dots in a halftone screen representation of a continuous tone image. For Amplitude Modulated (AM) screening, this involves shrinking a plurality of halftone dots located on a fixed periodic grid to a very small size, the density of the highlight being represented by the area of the dots. For Frequency Modulated (FM) screening, the size of the halftone dots is generally maintained at some fixed value, and the number of randomly or pseudo-randomly placed dots represent the density of the image. In both cases, it is necessary to print very small dot sizes to adequately represent the highlight areas.
Maintaining small dots on flexographic plates can be very difficult due to the nature of the platemaking process. In digital platemaking processes that use a UV-opaque mask layer, the combination of the mask and UV exposure produces relief dots that have a generally conical shape. The smallest of these dots are prone to being removed during processing, which means no ink is transferred to these areas during printing (the dot is not “held” on plate and/or on press). Alternatively, if the printing dots survive processing, they are susceptible to damage on press. For example small dots often fold over and/or partially break off during printing, causing either excess ink or no ink to be transferred.
Finally, photocurable resin compositions typically cure through radical polymerization, upon exposure to actinic radiation. However, the curing reaction can be inhibited by molecular oxygen, which is typically dissolved in the resin compositions and present in the general environment, because the oxygen reacts with the radical-producing compounds used to initiate polymerization in the photocurable layer. It is therefore desirable for oxygen to be excluded from the resin composition during image-wise exposure so that the photocurable resin composition can be more rapidly and uniformly cured.
One method of excluding oxygen involves placing the photosensitive resin plate in an atmosphere of inert gas, such as carbon dioxide gas or nitrogen gas, before exposure in order to displace the environmental oxygen. A noted drawback to this method is that it is inconvenient and cumbersome and requires a large space for the apparatus.
Another approach involves subjecting the plates to a preliminary exposure (i.e., “bump exposure”) of actinic radiation. During bump exposure, a low intensity “pre-exposure” dose of actinic radiation is used to sensitize the resin before the plate is subjected to the higher intensity main exposure dose of actinic radiation. The bump exposure is typically applied to the entire plate area and is a short, low dose exposure of the plate that reduces the concentration of oxygen, which inhibits photopolymerization of the plate (or other printing element) and aids in preserving fine features (i.e., highlight dots, fine lines, isolated dots, etc.) on the finished plate. However, the pre-sensitization step can also cause shadow tones to fill in, thereby reducing the tonal range of the halftones in the image. In the alternative, a selective preliminary exposure, as discussed for example in U.S. Patent Publication No. 2009/0043138 to Roberts et al., the subject matter of which is herein incorporated by reference in its entirety, has also been proposed. Other efforts to reduce the effects of oxygen on the photopolymerization process have involved special plate formulations alone or in combination with the bump exposure.
Finally, collimated light sources, where the collimation is achieved by optical or other mechanical means, have also been used to minimize the effect of oxygen on the photopolymerization process and alter the shape of resulting dots.
However all of these methods are still deficient in producing a relief image printing element having a superior dot structure for printing on various substrates. In addition, none of these methods allow for the dot shape to be tailored or modified in a directed fashion to satisfy the needs of the printing application.
Thus, there remains a need for an improved method of making a relief image printing element that allows for the tailoring or modification of the shape and/or geometric characteristics of the relief printing dots to provide superior performance for printing on various substrates and/or under various conditions.